Many reinforced concrete structures, which are exposed to the elements, will undergo a gradual change in internal chemistry in such a manner as to subject the internal steel reinforcing elements to corrosion. Under some circumstances, the concrete undergoes a slow carbonation process. As a result, the normally relatively high alkaline level of the concrete is progressively reduced. Eventually, the pH of the concrete reaches a level (around 9.5) at which it is no longer capable of preventing corrosion of the internal reinforcement.
Concrete can also become corrosive to its internal reinforcement where it is exposed to chloride infusion. This is common for roadways, bridge decks, parking garages, and the like, where chloride salts may be employed for controlling ice formation in the winter time. Over time, concrete exposed to chloride salts can become sufficiently infused therewith, that the internal steel reinforcement of the structure is subject to corrosion by reaction with the chlorides. In some cases, chlorides may be present in the concrete from the beginning, being used in some instances to hasten the process of the setting of freshly poured concrete.
In the above situations, if the condition is allowed to continue, serious damage to the concrete structure can be expected. When the internal reinforcement begins to corrode, it tends to expand, and the neighboring concrete is caused to crack and spall.
Procedures have been developed for rehabilitating concrete structures, to effect realkalization thereof or to reduce the chloride content, or both, depending upon the particular circumstances. The Vennesland et al. U.S. Pat. No. 4,832,803, for example, discloses an advantageous electro-chemical procedure for removing chlorides from mature concrete structures to significantly reduce corrosive conditions in and around the internal reinforcement. The Miller U.S. Pat. No. 4,865,702 discloses an electro-chemical process for realkalizing a mature concrete structure, also for the purpose of minimizing the corrosive activity of the concrete in the region of the internal reinforcement. The Miller U.S. Pat. No. 5,015,351 discloses certain electro-osmotic techniques which can be employed to the same end. Certain of the above-mentioned related pending applications are also directed to new and useful techniques for the rehabilitation of mature concrete structures by chloride removal and/or realkalization procedures.
In my prior copending U.S. patent application Ser. No. 539,069, filed Jun. 15, 1990, I disclose a rehabilitation procedure which involves application to an exposed surface of a concrete structure of a sprayed-on layer of self-adherent fibrous cellulosic pulp mixed with a liquid electrolyte. After applying a layer of desired thickness, a mesh-like electrode is placed over the matte of self-adherent pulp, after which a further covering layer of the self-adherent pulp is sprayed over the top of the electrode structure so that the electrode mesh is embedded in the fibrous matte. By maintaining an appropriate D.C. electrical potential between the embedded electrode structure and the internal reinforcement of the concrete, while maintaining the fibrous self-adherent pulp mass in a moist, electrolytically effective condition, it is possible to cause a migration of chloride out of the concrete, or to effect an infusion of hydroxyl molecules into the concrete, depending on whether the process being carried out is one of chloride removal or one of realkalization. The process according to my copending application is particularly advantageous in that the self-adherent fibrous pulp is easily applied to the structure, can be frequently remoistened as necessary, and is easily removed when process has been completed.
In accordance with the present invention, further significant improvements are made in the process and equipment disclosed in my copending U.S. patent application Ser. No. 539,069, enabling the procedures disclosed therein to be carried out even more expeditiously and economically.
Initially, the surface of the concrete to be treated must be prepared to receive the treatment. Typically, concrete that has reached the stage of requiring rehabilitation is already suffering from cracking, spalling and delamination. These defects are cleared and opened up to sound concrete by conventional methods. Pursuant to one aspect of the invention, cracks and voids are filled with a specially prepared mortar mixture having resistivity and capillarity compatible with the parent concrete. In addition, the mortar mixture is suitable for extended exposure to the electro-chemical environment while also accommodating the electrolytic action or electro-osmotic action which drives the rehabilitation process.
In accordance with known techniques, a distributed electrode structure is established over the face of the concrete to be treated. This electrode structure is embedded in an adherent electrolytic medium, advantageously a matte of cellulosic fibrous pulp, which is sprayed onto the surface to be treated as a mixture of the fibrous pulp material and a liquid electrolyte. A source of D.C. voltage is connected, one side to the distributed electrode structure embedded in the adherent coating, and the other terminal connected to the internal reinforcement of the concrete.
In accordance with one of the advantageous features of the present invention, a novel and improved form of distributed electrode structure is provided for placement on the external surface of the concrete to be treated. The new structure includes a pair of spaced-apart electrode supports, which may advantageously be in the form of elongated strips of wood, for example, of a thickness to support electrode elements at a suitable distance from the surface of the concrete. The electrode supports in turn mount a plurality of electrode support studs, which project outwardly from the supports at closely-spaced intervals. The studs are mounted on flexible mounting strips, which can easily be conformed to contoured surfaces. The mounting strip for at least one of the electrode supports, and usually only one, constitutes a conductive bus, by which each of the electrically conductive support studs can be connected to a voltage source. The distributed electrode is formed by a continuous electro-conductive element, which is threaded back and forth continuously from one electrode support to the other, being engaged at each end by one of the support lugs, so that the electrode element extends sinuously back and forth between opposed electrode supports, along the length of each of them, forming the desired distributed electrode structure.
After placing the electrode supports and installing the sinuous electrode element, the self-adherent electrolytic medium is sprayed onto the surface of the concrete, being sprayed through the installed distributed electrode. The sprayed medium is applied to a depth sufficient to fully cover and embed the distributed electrode.
To particular advantage, the sinuously installed continuous electrode element is in the form of a thin, flat, ribbon-like metallic strip, oriented on edge relative to the surface of the concrete being treated. The ribbon-like electrode strip thus presents a large conductive area to the electrolytic fiber mass in which it is embedded, while presenting minimum resistance to and interference with the sprayed-on application of the electrolytic mass following installation of the electrode.
The cellulosic pulp material employed in forming the self-adherent electrolytic mass is specially processed to facilitate its handling and application, and also to optimize the cosmetic aspects of the procedure, by minimizing staining and discoloration of the concrete.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment and practice of the invention, and to the accompanying drawings.